Because the iTunes Library is a database that can be accessed by a single client and Apple has not shown much interest to change and improve all this you currently can only make use of the iTunes Server of your NAS, if your NAS have any.To have the iTunes Library on a remote network location is just not a good idea. It can work, but it can give problems too. And especially it you are going to invoke the Library by multiple computers with iTunes on it.
So just better activate the NAS specific iTunes Server application and let it direct to the Library with all the music folders that you first have transferred to the NAS. Then you start iTunes with the Option key pressed to make sure you are going to let it point to a local Library as well and create a new Library. Later when iTunes is started you choose your NAS in the pulldown menu in the upper left corner.
A downside of all this is that your Library is not managed by the clients, it is play-only. But all clients can use and play from the same Library at the same time.
It is needless to say that this is also not an ideal situation. Let's hope for a better solution provided by Apple including a migration of current Library database in something better that has proper network capabilities.
